The present invention relates to the field of color printing and more specifically to the control of color printer actuation parameters. It discloses a computation model, computing systems and methods for computing ink thickness variations of color prints being generally printed with cyan, magenta, yellow and black inks. It represents an improvement over an initial model previously disclosed by one of the present inventors (see U.S. patent application Ser. No. 10/631,743, “Prediction model for color separation, calibration and control of printers”, filed Aug. 1, 2003, inventors R. D. Hersch, P. Emmel, F. Collaud).
Color control in printing presses is desirable in order to ensure that effectively printed colors correspond to the desired colors, i.e. the colors expected by the prepress color separation stage. Color consistency is desirable both across consecutive pages of a multi-page print job and also from print job to print job.
In the prior art, densitometers are often used to control the amount of ink of single ink printed patches. The densitometer measures the optical density, which is an approximate measure of the ink thickness. In the prior art, the control of printer actuation parameters affecting the printed output such as the ink thickness is generally performed by an operator or by an apparatus measuring the density of solid ink or of halftone ink patches, see U.S. Pat. 4,852,485 (Method of operating an autotypical color offset machine, Inventor F. Brunner, issued Aug. 1, 1989). Special patches are usually integrated along the borders of printed pages and serve as a means to measure their density. These special patches need however to be subsequently cut out.
Patent U.S. Pat. No. 4,685,139 (Inspecting device for print, to Masuda et. al, issued Aug. 4, 1987) teaches how to detect a print defect by comparing RGB sensor values acquired along a horizontal stripe perpendicular to the cylinder rotation orientation and pre-stored RGB sensor values. In the case that a defect is detected, an operator is called to take care of it.
U.S. Pat. No. 6,230,622 (Image data-oriented printing machine and method of operating the same, to P. Dilling, issued May 15 2001) teaches a method for operating a printing machine with an expert system which determines the effect of the interaction of a large number of print parameters and acts on some of these parameters in order to reach a high print quality. The proposed method relies only density measurements. Due to the large number of parameters which need to be taken into account, this solution seems very complex and costly.
U.S. patent application Ser. No. 10/631,743 (Prediction model for color separation, calibration and control of printers, inventors R. D. Hersch (also co-inventor in the present patent application), P. Emmel, F. Collaud, filed Aug. 1, 2003) teaches a method to deduce the ink thicknesses for a color patch printed with 2, 3 or 4 inks. The method works for deducing the ink thicknesses on single ink patches, on two ink patches and possibly on 3 ink patches. But due to the uncertainty between joint variations in the ink thicknesses of cyan, magenta and yellow and a variation in thickness of black, the method does not work well for the set of cyan, magenta, yellow and black inks. In addition, since spectral measurements are performed on specific chromatic halftone elements within a printed page, the method does not easily allow performing real-time control of ink thicknesses on high-speed printing presses. Finally, the proposed way of computing scalar ink thicknesses assumes that the inks do not scatter back light, i.e. that they do not penetrate into the paper bulk.
U.S. patent application Ser. No. 10/698,667 (Inks Thickness Consistency in Digital Printing Presses, to Staelin et al., filed Oct. 31, 2003) teaches a model for estimating ink thickness control parameters such as the developer voltage in case of an electrographic printer. This model takes as input values the densities of monochrome patches. This patent application does neither teach how to obtain ink thickness control parameters from polychromatic halftone patches nor from halftones being part of the actual printed pages.
U.S. patent application Ser. No. 10/186,590 (Measurement and regulation of inking in web printing, to Riepenhoff, also co-inventor of the present application, filed 1Jul. 2002) teaches a process for measuring the mean spectrum integrated over a stripe of the printed page. It also teaches a device for regulating the ink density by predicting the mean reflection spectrum along a stripe thanks to a correspondence function between image data located along the stripe and the resulting reflection spectrum. However, that correspondence function does not incorporate an explicit ink thickness variable, nor does it make the distinction between nominal surface coverages and effective surface coverages. It therefore does not account for the ink spreading phenomenon. Finally, that application does not teach how to take into account the uncertainty between joint variations in the densities of the cyan, magenta and yellow inks, and a variation in the density of the black ink.
The present disclosure solves the above mentioned problems and provides a stable means of deducing in real time ink thickness variations of cyan, magenta, yellow and black on a running printing press or printer, without needing special solid or halftone patches within the printed page.